Overhoist limit switch

ABSTRACT

An overhoist limit switch including a base having a number of fixed contacts supported thereon and corresponding number of movable contacts mounted on a trip shaft which is journaled for oscillatory motion on the base, an operating shaft journaled in the housing and including a weight arm which responds to overhoist movement, and a trip assembly including a trip plate fixedly secured to the trip shaft and having a cam follower, a rocker arm fixedly secured to the operating shaft, a cam arm pivotally connected to the rocker arm and having an inverted cam positioned to engage the cam follower and a spring to provide a snap action force through the cam arm to the trip plate.

United States Patent [72] lnventor Herbert Kupsis Milwaukee, Wis. [21] Appl. No. 15,505 [22] Filed Mar. 2, 1970 [45] Patented Oct. 19, 1971 [7 3] Assignee l-larnischfeger Corporation West Milwaukee, Wis.

[54] OVERIIOIST LIMIT SWITCH 6 Claims, 8 Drawing Figs.

[52] US. Cl 200/47, 200/153 C, 200/153 SC [51] Int. Cl "01h 5/10 [50] Field of Search ..200/47, 153 G, 153 C, 153 SC, 67G

[56] References Cited UNITED STATES PATENTS 927,417 7/1909 Libby 200/47 1,113,592 10/1914 Wright et al. 200/47 1,454,827 5/1923 Beck 200/47 1,509,685 9/1924 Moore.. 200/47 X 1,560,057 11/1925 Hunter 200/47 X Primary Examiner-Robert K. Schaefer Assistant Examiner-William J. Smith Attorney.lames E. Nilles ABSTRACT: An overhoist limit switch including a base having a number of fixed contacts supported thereon and corresponding number of movable contacts mounted on a trip shaft which is journaled for oscillatory motion on the base, an operating shaft journaled in the housing and including a weight arm which responds to overhoist movement, and a trip assembly including a trip plate fixedly secured to the trip shaft and having a cam follower, a rocker arm fixedly secured to the operating shaft, a cam arm pivotally connected to the rocker arm and having an inverted cam positioned to engage the cam follower and a spring to provide a snap action force through the cam arm to the trip plate.

IZL L PATENTEDUCT 191911 SHEET 1 BF 4 PATENTEDHCI 1 9 19 3.6 l 4. 349

I sum 2 or 4 PATENTEUnm 19 \sn sum u 0F 4 ovsnuors'r LIMIT SWITCH BACKGROUND OF THE INVENTION The present invention pertains to overhoist limit switches which are used to limit the operation of an electric motor whenever the motion of a hoist exceeds a predetermined limit through mechanical error or an electrical failure. This is of particular significance in a hoist where an override may damage the hoist cables. 4

A number of such limit switches have been used to prevent this from happening, such as shown in the U.S. Pat. No. 3,315,045, issued Apr. I9, 1967 and entitled Overhoist Limit Switch. This type of a switch relies on the force of a torsion spring to provide snap action motion of the trip shaft. The torsion spring force is reduced considerably as it approaches zero force and therefore must be preloaded considerably to assure a constant force through the full distance of movement.

SUMMARY OF THE PRESENT INVENTION The overhoist limit switch of the present invention provides positive control of the operation of a hoist motor whenever an override condition occurs. This is accomplished by using an inverted cam as a transfer element between an actuating spring and a trip shaft on which the movable contacts are mounted. The inverted cam permits unlimited design flexibility in the trip shaft torque provisions. The torque applied to the trip shaft and to the electrical contacts can be selected to be a constant or a variable depending on the requirements to obtain the most efficient electrical arc interruption. along with optimum mechanical construction.

The inverted cam allows the trip shaft to trip in the same direction as the operating shaft. This makes it possible, without using any complicated leverages or linkages, to engage the operating shaft directly to the trip shaft and provide an additional impact force to open the contact tips in case they fail to open.

Both the trip actuating force and the switch contact holding force is provided by a single compression spring. This is possible again due to the use of an inverted cam. The cam surface which is used to apply the trip force in one direction is also used to apply the holding force in the same direction.

Other objects and advantages will appear as this disclosure progresses, reference being had to the following detailed description when read in connection with the accompanying drawings.

A BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the overhoist limit switch of this invention with a portion broken away to show the trip assembly and the electric switches.

FIG. 2 is atop view of the switch with the cover removed;

FIG. 3 is a view taken along line 3-3 of FIG. 2 showing one pole of the electric switches.

FIG. 4 is a view taken on line 44 of FIG. 2;

FIG. 5 is a view in section taken on line 5-5 of FIG. 2, showing the normal position of the actuating assembly;

FIG. 6 is a view similar to FIG. 5 showing the rocker arm partially rotated to the trip position;

FIG. 7 is a view similar to FIG. 5 showing the rocker arm in the trip position; and

FIG. 8 is a view similar to FIG. 5 showing the rocker arm in the tripped position.

DESCRIPTION OF A PREFERRED EMBODIMENT The overhoist limit switch 10 of this invention as seen in FIG. 1 generally includes a support or base member 12 and an insulating cover 14 secured to the support plate by means of latches or clasps 16. An operating shaft 18 is journaled for oscillatory motion in a set of ball bearings. 19 provided in a bearing housing 21 which is secured to one end wall of the support member 12. The operating shaft 18 is rotated in response to an overhoist condition by means of a weight arm 22 which is secured to the operating shaft 18 and has an integral weight 24 at one end and a hanging weight 26 secured to the other end by means of a cable 28. The rotary motion of the weight arm 22 is limited by means of stop pins 30 and 32 provided on the base member 12. The rotary motion of the operating shaft 18 is used to actuate a trip assembly 60 to open or close a number of conventional electric hoist motor switches 35 provided on the support plate 12.

The electric motor switches 35 are formed by a number of movable contacts 34 mounted on a trip shaft 36 and a number of fixed contacts 38 mounted on the base member 12. More particularly and referring to FIGS. 2, 3, and 4, the trip shaft 36 is shown journaled in ball bearing 40 provided in a bearing plate 42 secured to the other end wall 44 on the support plate 12 and a ball bearing 46 provided in the end of the operating shaft 18. Three sets of movable contacts 34 are shown mounted on the trip shaft 36. A corresponding number of fixed contacts 38 are provided on the support member 12 on one side of the movable contacts 34 and a corresponding number of fixed stops 37 are provided on the support member 12 on the other side of the movable contacts 34. Conventional arc chutes 48 are provided for each switch 35 formed by a set of movable contacts 34 and fixed contacts 38.

Referring specifically to FIG. 3, it will be noted that means are provided for biasing each of the movable contacts 34 into engagement with the fixed contacts 38. Each of the contacts 34 is mounted on one of a pair of support arms 50 which are pivotally connected to a bracket 52 on the trip shaft 36 by means of bolts or pins 54. The support arms 50 are biased outwardly from each other by means of a spring 56 and are limited in movement by a flat section 58 which engages the bracket 52. The spring 56 allows for a small amount of movement of the movable contact 34 to assure positive contact with the fixed contact 38. As shown in the drawing, this limit switch is designed for three pole interruption of three-phase AC power. The three poles provide additional switching capability since every circuit in a three-phase system is interrupted with two contacts always in series. Although three poles are shown, it should be apparent that this limit switch can be readily adapted to two pole AC operation or two pole double throw DC operation, if desired. Additional switching capability can also be provided by mounting switch contacts on the fixed stops 37 and movable contacts on the other support arm 50.

The limit switch 10 is normally in the position shown in FIGS. 1, 3 and 5 with the weight 26 hanging freely from the weight arm 22 and the weight arm 22 resting on the limit stop pin 30. Whenever the weight 26 is moved upward by the engagement with a hoist or hook, the mass of the integral weight 24 will rotate the shaft 18 counterclockwise in FIG. 1 and clockwise in FIG. 5 until the weight 24 rests on the second limit stop pin 32. In accordance with the invention, the motion of the operating shaft 18 is imparted to the trip shaft 36 by means of the snap action trip assembly 60. In this regard, the assembly 60 as shown in FIGS. 4 and 5 includes a trip plate 62 secured to the trip shaft 36 and a rocker arm 64 secured to the operating shaft 18. The trip plate 62 includes a cam follower 66 journaled for rotary motion on the center arm 68 of the trip plate 62 and a pair of cam rollers 70 journaled for rotary motion on the outwardly extending arms 72 of the trip plate 62. The rocker arm 64 includes spring plate 74 on one end and a bifurcated boss 76 on the other end. A pair of latch release pins 78 are provided on the lower end of the rocker arm 64.

Means are mounted on the rocker arm 64 for transferring or imparting a trip force to the trip plate 62 whenever the operating shaft 18 moves a predetermined distance relative to the trip shaft 36. Such means is in the form of a cam arm 80 pivotally connected to the boss 76 on the rocker arm 64 by a pivot pin 82 and an actuating spring 84. The cam arm 80 includes a spring plate 86 at the other end and an inverted cam 88 intermediate the ends of the cam arm. The inverted cam 88 includes a clockwise or first cam surface 90 and a counterclockwise or second cam surface 92 and is positioned to engage the cam follower 66 provided on the trip plate 62. The

actuating spring 84 is positioned between the spring plates 74 and 86 to bias the cam arm 80 away from the rocker arm 64. The direction of the force imparted to the trip plate 62, clockwise or counterclockwise, will depend on the position of the cam follower 66 in the inverted cam 88 when the trip force is released as described more fully below.

Means are provided for restraining the motion of the trip plate 62 on movement of the operating shaft 18 until sufficient force is built up in the spring 84 to produce a snap action force in the movement of the trip shaft 36. Such means is in the form of a pair of latch assemblies 94R and 94L mounted on the base plate I2. Each of the latch assemblies 94 includes a latch arm 96 pivotally connected to the base 12 and having an arcuate cam surface 98 and a release arm 100. The latch arms 96 are biased by means of spring 102 into engagement with the cam rollers 70 provided on the latch plate 62. One or the other of the latch arms 96 will hold the trip plate 62 fixed until one of the release pins 78 on the rocker arm 64 engages one of the release arms 100.

The limit switch is normally in the position shown in FIG. I with the weight 26 holding the weight arm 22 against the limit stop 30. When the weight 26 begins to rise, the weight 24 will start to rotate the shaft 18 relative to the trip shaft 36. The holding force of the spring 84 acting on the trip shaft 36 in the normal position will be reversed and a trip force will be stored up in the spring 84. When the trip plate 62 is released, the trip force which has been stored up in the spring 84 will snap the trip plate 62 and trip shaft 36 to the other side opening electric switches 35. The final movement of the weight 24 into engagement with the stop 32 will store up a holding force in the spring 84 to hold the trip shaft 36 in the open position.

SEQUENCE OF OPERATION In the normal position of the trip shaft 36 as seen in FIG. 5, the trip plate 62 is rotated counterclockwise to an initial position and is locked in position by means of the latch assembly 94L to hold the trip plate 62 in position. The rocker arm 64 will also be rotated counterclockwise with the cam follower 66 on the trip plate 62 seated in the right-hand cam surface 92 on the inverted cam 88 and the cam arm 80 pivoted or squeezed toward the rocker arm 64. The spring 84 will be squeezed between the spring plates 74 and '86 to provide a counterclockwise holding force to the trip shaft 36 to hold the movable contacts 34 against the fixed contacts 38.

Whenever the weight 26 is moved upward by the overrice motion of a hoist, the operating shaft 18 will initially rotate the rocker arm 64 to the position shown in FIG. 6. The cam roller 66 will be seated in the center of the inverted cam 88. The force of the spring 84 will be neutralized so that'no rotary force or torque acts on the trip plate 62. However, the trip plate 62 will still be locked in the latched position by the latch assembly 94L holding the switch 35 closed.

As the operating shaft 18 and rocker arm 64 continue to move clockwise to the position shown in FIG. 7, the cam surface 90 will move under the cam roller 66 rotating the cam arm 80 toward the rocker arm 64. The actuating spring 84 will be squeezed between the spring plates 74 and 86 building up the spring force of the spring 84. The spring force stored in the spring 84 is released when the release pin 78 on the rocker arm 64 engages the release arm 100 on the latch arm 96 and rotates the latch arm 96 far enough to clear the roller 70 on the trip plate 72. The force of the spring 84 when released will act through thecam surface 90 and cam follower 66 to snap the trip shaft 36 clockwise and move the movable contacts 34 away from the fixed contacts 38 and into engagement with the fixed stop 37 on the opposite side of the base member 12.

As seen in FIG. 8, the trip plate 62 will rotate far enough to lock the roller 70 in the latch assembly 94R; The integral weight 24 will move into engagement with the limit stop pin 32 and the cam roller 66 will be seated in the left hand end of the inverted cam 88. The spring 84 will again be squeezed between the spring plates and the cam surface 90 storing up a holding force which acts against the cam follower 66.

When the weight 26 again drops to a normal position rotating the operating shaft 18 clockwise (FIG. I) the same sequence of events will occur in the transfer of the force of the operating shaft 18 to the trip shaft 36. The rocker arm 64 will rotate counterclockwise moving the cam 88 under the cam roller 66 until the pin 78 engages release arm 100 on latch assembly 94R and releases the trip plate 62. The force of the actuating spring 84 will act through cam surface 92 to snap the trip plate 62 andtrip shaft 36 counterclockwise until the cam roller 70 again is locked in latch assembly 94L. The movable contacts 34 will close on the fixed contacts 38 completing the circuit through the switches 35.

Means are provided for imparting a positive mechanical force to the trip plate 62 in the event the movable contact 34 has become welded to the fixed contact 38 due to arcing during the closing motion of the switch 35. Such means is in the form of the inverted cam surface 88 which acts against the cam roller 66. Referring to FIG. 7, it should be noted that the left end 91 of the inverted cam 88 is mechanically engaged with the cam roller 66. If the trip plate 62 does not move on release of the latch assembly 94, the mass of the weight 24 will act directly on the trip plate 62 with sufficient torque to break any minor welds which may have occurred between the electrical contacts. The same mechanical action will occur on reverse motion of the operating shaft due to the engagement of the right end 93 and the inverted cam 88 with the cam roller 66.

The torque imparted to the trip plate 62 and trip shaft 36 can be preselected to be a constant or a variable by means of the contour of the cam surfaces 90 and 92. This is particularly significant since different switches require different opening and closing forces to achieve the most efficient electrical arc interruption; In the present embodiment, a constant tripping torque is provided in both directions of movement by using an inverted cam which can be formed to compensate for variations in spring force, cam arm length and directionof force.

RESUME The overhoist limit switch 10 as described above provides full spring force throughout the full tripping motion of the trip shaft because of the use of an inverted cam. This force may be constant throughout the movement of the trip shaft or variable depending on the shape of the cam surfaces and 92. It should be understood that the opening and closing forces for AC or DC operation varies and that this variation can be compensated for by the appropriate cam contour. Positive movement of the trip shaft is also assured because of the mechanical engagement of the operating shaft with the trip shaft. This limit switch can be used to control two or three pole systems because of the versatility of the forces available in using an inverted cam.

lclaim:

1. An overhoist limit switch comprising a base member,

an operating shaft journaled for rotation on said base member,

a trip shaft joumaled in said operating shaft and on said base member,

a plurality of fixed electrical contacts secured to said base member,

a plurality of movable electrical contacts secured to said trip shaft for movement into engagement with said fixed contacts, I v

means for imparting snap motion to the movement of said trip shaft in response to a predetermined movement of said operating shaft in the same direction, I

said snap motion imparting means including a trip plate secured to said trip shaft and having a cam follower, a rocker arm secured to said operating shaft, a cam arm pivotally connected at one end to said rocker arm and having an inverted cam positioned to engage said cam follower and an actuating spring positioned between the other end of said rocker arm and said cam arm to bias said cam against said cam follower, and

means for restraining the motion of said trip plate until sufficientforce has been built up in said spring to snap the trip plate to the other side.

2. The switch according to claim 1 wherein said imparting means includes a first cam surface for imparting a force to said trip plate in one direction and a second cam surface for imparting a force to said trip plate in the other direction.

3. The switch according to claim 1 including means on said rocker arm for releasing said restraining means.

4. The switch according to claim 1 wherein said snap motion imparting means includes means for positively moving said trip shaft after said operating shaft has moved a preset distance.

5. An overhoist limit switch comprising:

a base having end walls,

a number of fixed electrical contacts mounted on said base,

an operating shaft journaled for rotation in one of said end walls and means for rotating said operating shaft in response to an overhoist condition,

a trip shaft having one end journaled for rotation in the other of said end walls and the other end journaled for rotation in said operating shaft,

a number of movable contacts mounted on said trip shaft in a position to engage said fixed contacts,

a trip plate secured to said trip shaft,

a rocker arm secured to said operating shaft,

means for transferring the motion of said rocker arm to said trip plate after said operating shaft has rotated a predetermined distance relative to said trip shaft, said transferring means including a cam arm pivotally mounted on said rocker arm, an inverted cam having a first cam surface for moving said trip shaft in one direction and a second cam surface for moving said trip plate in the other direction and a compression spring between said cam arm and rocker arm to provide a trip force to the movement of said trip shaft, and

means for restraining the motion of said trip plate until sufficient force has been built up to produce a snap motion in the movement of said trip plate.

6. An overhoist limit switch comprising:

a support member,

a number of fixed contacts mounted on said support member,

an operating shaft journaled for rotation on said support member,

a weight arm connected to said operating shaft to provide rotary motion in response to an overhoist condition,

a trip shaft mounted for rotary motion on said support member and having a trip plate secured thereto,

a cam follower mounted on said trip plate,

a rocker arm secured to said operating shaft,

a cam arm pivotally connected to said rocker arm and having an inverted cam positioned to engage said cam follower,

a compression spring positioned between said cam arm and said rocker arm to bias said cam arm against said cam fol lower,

said inverted cam including a first cam surface positioned to move into engagement with said cam follower when said operating shaft is moved in one direction and a second cam surface positioned to move into engagement with said cam follower when said operating shaft moves in the other direction,

said cam arm being moved toward said rocker arm to compress said spring by the movement of the cam follower into engagement with said cam surfaces whereby the force stored up in said spring will provide a rotary force to said trip shaft through said trip plate, and

means for restraining the motion of said trip shaft until sufficient force is built up in said spring to produce a snap movement in the motion of said trip shaft. 

1. An overhoist limit switch comprising a base member, an operating shaft journaled for rotation on said base member, a trip shaft journaled in said operating shaft and on said base member, a plurality of fixed electrical contacts secured to said base member, a plurality of movable electrical contacts secured to said trip shaft for movement into engagement with said fixed contacts, means for imparting snap motion to the movement of said trip shaft in response to a predetermined movement of said operating shaft in the same direction, said snap motion imparting means including a trip plate secured to said trip shaft and having a cam follower, a rocker arm secured to said operating shaft, a cam arm pivotally connected at one end to said rocker arm and having an inverted cam positioned to engage said cam follower and an actuating spring positioned between the other end of said rocker arm and said cam arm to bias said cam against said cam follower, and means for restraining the motion of said trip plate until sufficient force has been built up in said spring to snap the trip plate to the other side.
 2. The switch according to claim 1 wherein said imparting means includes a first cam surface for imparting a force to said trip plate in one direction and a second cam surface for imparting a force to said trip plate in the other direction.
 3. The switch according to claim 1 including means on said rocker arm for releasing said restraining means.
 4. The switch according to claim 1 wherein said snap motion imparting means includes means for positively moving said trip shaft after said operating shaft has moved a preset distance.
 5. An overhoist limit switch comprising: a base having end walls, a number of fixed electrical contacts mounted on said base, an operating shaft journaled for rotation in one of said end walls and means for rotating said operating shaft in response to an overhoist condition, a trip shaft having one end journaled for rotation in the other of said end walls and the other end journaled for rotation in said operating shaft, a number of movable contacts mounted on said trip shaft in a position to engage said fixed contacts, a trip plate secured to said trip shaft, a rocker arm secured to said operating shaft, means for transferring the motion of said rocker arm to said trip plate after said operating shaft has rotated a predetermined distance relative to said trip shaft, said transferring means including a cam arm pivotally mounted on said rocker arm, an inverted cam having a first cam surface for moving said trip shaft in one direction and a second cam surface for moving said trip plate in the other direction and a compression spring between said cam arm and rocker arm to provide a trip force to the movement of said trip shaft, and means for restraining the motion of said trip plate until sufficient force has been built up to produce a snap motion in the movement of said trip plate.
 6. An overhoist limit switch comprising: a support member, a number of fixed contacts mounted on said support member, an Operating shaft journaled for rotation on said support member, a weight arm connected to said operating shaft to provide rotary motion in response to an overhoist condition, a trip shaft mounted for rotary motion on said support member and having a trip plate secured thereto, a cam follower mounted on said trip plate, a rocker arm secured to said operating shaft, a cam arm pivotally connected to said rocker arm and having an inverted cam positioned to engage said cam follower, a compression spring positioned between said cam arm and said rocker arm to bias said cam arm against said cam follower, said inverted cam including a first cam surface positioned to move into engagement with said cam follower when said operating shaft is moved in one direction and a second cam surface positioned to move into engagement with said cam follower when said operating shaft moves in the other direction, said cam arm being moved toward said rocker arm to compress said spring by the movement of the cam follower into engagement with said cam surfaces whereby the force stored up in said spring will provide a rotary force to said trip shaft through said trip plate, and means for restraining the motion of said trip shaft until sufficient force is built up in said spring to produce a snap movement in the motion of said trip shaft. 